Transcript Processing of Thermoplastics

Processing of Thermoplastics
1. Extrusion
• What is extrusion?
– The word extrusion comes from Greek roots-means
‘push out’
– Continuous process
– Process which forcing a molten materials (plastic)
through a shaped die by means of pressure- e.g.
melting of plastic resin + adding mixing fillers
– In this process, screws are used to progress the
polymer in the molten or rubbery state along the
barrel of the machine
– Single screw extruder is widely used, however twin
screw extruder are also used where superior mixing is
needed
Main features of a single screw
extruder
The channel depth decreases from
feed end to die end
Solid polymer is fed in at one end, inside the polymer melts and
Homogenizes and molten extrudate emerges from the other
There are 3 zones; feed zone, compression zone and metering zone
Typical extrusion line showing
major equipment
Materials fed into hopper, falls through a hole in the top the extruder (feed throat)
Onto the screw. The screw moves the molten plastic forward until the end of the
Extruder barrel to which die has been attached. Die gives shape to molten plastics,
Cooled in water tank.
Equipment of Extruder
• Drive motor- turns the screw, provides
power for the operation of the extruder to
push out the plastic materials
– The required extruder power increases when;
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Output increases
Barrel diameter increases
Screw length increases
High output is required at high temperature
Power requirement is a function of resin type and mold design
Equipment of Extruder
• A large thrust bearing- mounted on the screw.
Prevent the screw from moving backwards
• Barrel- is the chamber in which the screw turns
and the resin flows (made of hardened steel.
– The inside diameter of barrel indicates the capacity &
size of extruder.
– Outside of barrel is jacketed with electrical heating
element
– Heating elements are divided into different controlled
zones
Equipment of Extruder
• Feed throat- opening in the top of the
barrel, just beyond the thrust bearing (Inlet
for the resin)
• Hopper- mounted over the feed throat
• Extruder screw- attached to the drive
linkage through the thrust bearing and
rotates inside the barrel
Functions of Extruder Screw
– To convey the resin through the extruder
– To mix the ingredient together
– To build pressure in the extruder (so that resin
will be pushed through the die)
– To impart mechanical energy as part of the
melting process
Extruder screw
The screw is machined out of a solid rod.
Like a shaft with helical screw on it,
each turn of the helix is called a flight.
Important parameter= L/D of the screw (length of the flighted portion of the
screw/ inside diameter of the barrel)
L/D measures the capability of the screw to mix materials and ability of the
screw to melt hard-to-melt material. Typical L/D ratios are 16:1 to 32:1
Extruder screw
• Barrel diameter is constant
over the entire length of the
extruder
• The root is the measure of the
diameter of the shaft of the
screw (the root diameter can
vary along the length of screw)
• The flight rise above the shaft
creating a flight depth
(difference between top of the
flight and the root diameter)
• As the root diameter changes,
the flight depth will
correspondingly change (if the
root diameter is small, the flight
depth are large and vice versa
Zones in a single screw extruder
The channel depth decreases from
feed end to die end
Decreasing in channel depth
results in increasing pressure along
the extruder
Feed Zone
• Purpose; Preheat the polymer, and convey
it to subsequent zones
• Pulls the polymer pellets from the hopper
• The screw depth is constant
• The feed section has a small, constant
root diameter that results in large,
constant-depth flight to accommodate the
bulky dry solid resins and other additives
Compression Zone
• The second zone- decreasing channel
depth
• Usually called as ‘compression’ and
‘transition’ zone
• Compresses the material conveys from
the feed zone and plasticates it
• Can be identified as by the gradual
increase in the diameter of the root along
the length of the section
Compression Zone
• root diameter increase means the flight depth
gradually decrease throughout the compression
section, compressing the resin and forcing the
air/volatiles out of the resin melt
• The volatiles escape by flowing backward
through the vent port or gap between screw and
barrel
• Removal of these volatiles is important in
making pore/void- free product
The Die Zone
• Located in this region is the screen pack
(comprises a perforated steel plate called
breaker plate and sieve pack)
• The breaker plate-screen pack has three
functions;
– To sieve out/remove unwanted particles, e.g. dirt,
foreign bodies (dies are expansive and difficult to
repair)
– To develop a head pressure that provides the driving
force for the die
– To remove ‘turning memory along the spiral screw’
from the melt (Polymers are made up of long chain
molecules, coiled, etc. , they have tendency towards
elastic recovery)
An example of ‘turning memory’
• New design of
flooring block; highlyfilled PVC compound
(PVC + plasticiser +
CaCO3, heat
stabilizer + pigment)
• After the tiles were
removed from the
cooling bath, they
were all twisted
(result from turning
memory from the
screw).
• The breaker plates is
introduced in the
extruder to break up
the plug of polymer
containing the
aligned ‘memory’
New design of flooring block
Manufacturing of parquet flooring blocks
Metering zone
• Constant screw depth and very shallow flight
depth
• The function is to homogenize the melt and
supply to die region (give final mixing)
• Shallow flight depth ensure that high shear is
added to the resin to accomplish any melting of
the residual solids.
• High shear also builds pressure on the melted
resin and push out of the end of the extruder
Important extrusion parameter; Compression Ratio (measures of the work that is
Expanded on the resin)
Compression Ratio = flight depth in the feed section / flight depth in metering section
(as low as 1.1/1 and as high as 5:1, typically 2.25:1)
Special Screw Design
• Modification of the screw basic design is
needed to obtain good distribution of filler
• However changing screw design is a
difficult task, thus general purpose screw
is used
• The performance of these general purpose
screw can be modified by changes in
operational setting such as temperature,
screw speed, etc
The Screw
• The screw is the heart of an extruder
• The geometry of the screw changes along
the length
Common screw geometry, with three-zone screw is the most common
Variation in Screw Design
• PE, e.g. LDPE melts
gradually- screw with
overall length evenly
divided between three
zones (PE screw)
• If the polymer melts
sharply, very short
compression zone is
needed (nylon screw)
• PVC, its melting is more
gradually than PE
(difficult to extrude)- use
a screw with one long
compression zone
along its entire length
• General purpose screw- the performance
of this extruder can be modified by
changes in setting; temperature, screw
speed, etc.
Temperature profiles for PE and nylon when extruded with
general- purpose screws versus resin-specific screws
• Mixing of two or more resins are strongly
dependent upon viscosities (materials are
mixed more efficient when viscosities are
similar, e.g. temperature mixing for PMMA
and PE is at 218C)
Plot of viscosities
of common resins as
a function of
temperature
Head Zone
• Portion of extruder follows the end of screw
• After leaving the end of screw, plastic flow
through screen pack then through breaker plate
(disc of sturdy metal with many holes drilled
through it)
• Screen pack – collection of wire screen (usually
in different mesh), to filter out unmelted resin or
contaminants
• Screen pack will become clogged with filtered
materials and must be changed (at this point, is
said to be blinded). It is noted by an increase in
the back pressure in the extruder
Head Zone
Head zone and typical die
Die
• The shaping tool that is mounted on the end of
extruder; onto a ring called adapter
• Purpose; to give shape to the melt
• Most extrusion dies made of stainless steel
Die used for making a rod
Cooling
• Upon exiting the die, the extrudate must
be cooled to retain its shape
• The extrudate is introduce into a cooling
bath, extrudate can passes through sizing
plate (plates of rings with holes of the
proper size)
Puller
• After the part has been colled, it will retain
its shape under moderate tension and
radial compression force, then enter a
puller
• Puller is required to draw the materials
away from the extruder
Twin-Screw Extruder
• Can be divided into co-rotating and
counter-rotating types
• Twin-screw extruder is a relatively
expensive machine;
– Difficult to accommodate bearings
(dimensions limited)
– Complicated gear boxes
– Two screws
Twin-Screw Extruder
The screw rotate in the
same direction
The screw rotate counter
To each other
Twin-Screw Extruder-Corotating
• Corotating – the material is passed from
one screw to another and follows a path
over and under a screw
• The path ensures that most of the resin
will be subjected to the same amount of
shear as it passes between screw and
barrel
Twin-Screw ExtruderCounterrotating
• Material is brought to the junction of the
two screws and material bank is build up
on top of the junction
• This build up of the material is conveyed
along the length of the screw by the screw
flights
• Total shear is lower than in single-screw
and corotating twin screw
Corotating vs Counterrotating
• Which of these methods produce better
mixing? Why?
Start-up
• The extruder should be preheated before
attempting to turn the screw (heating zones and
die)
• When some resins are used in extrusion (
especially those likely to decompose with
prolong heating), the resins are removed from
the extruder by running another resin through
the extruder before shutdown- this process is
called purging
• Purging resin should be easy to melt, have
sufficient density to sweep the prior resin, be
known to present no start-up problem
Capacity
• The single most important parameter that
determine extruder capacity is the size of
screw
• Total flow of the extruder (total amount of
extruder that passes through the extruder)
• Total flow = drag flow – pressure flow –
leakage flow
Capacity
• Drag flow = measure of the amount of
material that is dragged through the
extruder by friction action by the barrel anf
the screw
• Pressure flow = flow that is caused by the
back pressure inside the extruder
• Leakage flow = the amount of materials
that leaks past the screw in the small
space between the screw and the barrel
Drag Flow
• Determined by a consideration of flow between parallel
plate in a classical analysis of Newtonian fluid flow
• Drag flow = (1/2)π2D2NHsinθcosθ
– D = diameter of the screw
– N = speed of the screw
– H = flight depth in the metering section
– Θ = pitch angle
Flow in extruder increase by increasing the diameter of the screw,
Increasing the speed of the screw, and increasing the flight depth
Pressure Flow
• Can also be found by classical Newtonian
flow analysis
• Pressure Flow = πDH3Psin2θ
•
12ηL
• Total Flow = (1/2)π2D2NHsinθcosθ - πDH3Psin2θ
•
12ηL
Total Flow
• Screw dimensional parameter; D, H, θ, L and the other
constant are combined into two constant, α and β
• Total Flow = αN – (βP/η)
– Increasing in the speed of extruder (N) will increase the output of
a particular screw
– Output of extruder will decreased by increase in the back
pressure (P)
– Back pressure will increase significantly as the screen pack
become contaminated
– If the viscosity decreases, as it would when the temperature is
increased, the second term of equation will increase, and
decrease the output
Part Dimension Control
• The geometry of the die is the major influence
on setting the part size and shape
• Important phenomena that occurs in this region
is the swelling of the size (cross section) of the
extrudate as it exits the die
• The swelling is called ‘die swell’
• The die swell is measured as the ratio of the
diameter of the extrudate to the die orifice
diameter (Dx/Dd) after exiting the die
Die Swell
• The effect in which the polymer swells as it
leaves the die
• The result is an extrudate which differs in its
dimensions from those of the die orifice
• Die well results from recovery of the elastic
deformation as the extrudate leaves the die
channel before it freezes
Dd
Dx
Die swell in (a) rod and (b) pipes
Die Swell
• Is caused by the viscoelastic nature of the
polymer melt (also has been called as ‘plastic
memory’-as it restore the shape previously held)
• Die swell can be reduced by;
– Extending the land
– Increasing temperature- impart the energy needed to
disentangled the molecules
– Shortened the distance between the die and the
water tank
Defects- Melt Fracture
• Melt fracture- Skin rupture usually occurs only on the
outside surface of the film when stretching and cooling
occur too fast and cause micro tears.
• Melt fracture caused by skin rupture occurs when the
surface of the film is stretched too quickly on leaving the
die.
• the extrudate has a rough surface, with short cracks that
are oriented at the machine direction or helically around
the the extrudate.
• Occur due to low temperature of the melt, high molecular
weight, die is not properly streamlined, etc
• Solve by; streamlined the die, raising the melt
temperature, selecting resin with low molecular weight,
etc.
Defects- Melt Fracture
Effect of streamlined in a die to prevent melt fracture
Defects
• Die exit instability
– shark skin– the outer surface
of the part is rough with line
running perpendicular to the
flow direction (a tearing of the
outer surface- usually
associated with stresses in the
extrudate from sticking to the
die wall)
– orange peel- defect in a
surface of an extrudate in
which a small dimple are
formed
– Bambooing- defect in a
surface of an extrudate that
resembles bamboo
Defects- Degradation
• Detected by discolorations and lower
physical and mechanical properties
• Caused by; too high heat for the speed of
the extrusion, past resin that not fully
purged, etc
• Solved by; good combination of heat and
extrusion speed, better purging
materials/procedures, etc.
Defects- Contamination
• Detected by sports (small dimples) in the
extrudate- sometimes called ‘eye-fish’
• Caused by; contamination (dust, other
resin) fall into the hopper or other parts of
resin conveying system
• Solved by; keep hopper covered,
inspecting the incoming materials, etc
Defects- Bubbles in the Extrudate
• Excessive moisture/volatiles can be
absorbed by resin and then vaporized
when the melt exits the die- resulting
bubbling in the extrudate
• Solved by; dry the resin before fed into the
hopper, store the resin in low humidity
location, etc.